In the engineering world, mission critical facilities can refer to a variety of different types of facilities. The first type of facility that tends to come to mind is a data center because the "mission" of the facility is critical and continuity of operations is essential.
However, from a life safety perspective, mission critical often refers to a facility in which the mission or operation of the facility is critical and the prompt evacuation of the facility is not possible. This can include health care facilities, detention and correctional facilities, and industrial facilities in which at least some operation or control personnel need to remain in place for a period of time.
Mission critical industrial occupancies
In December 1992, three workers at the O'Brian Newark Cogenerating Plant (Newark, N.J.) perished in a fire when they could not find a safe egress path. The employees were in a control center on the second floor with one means of egress, which was through the area where the fire occurred. The control center had exterior windows, but the employees were not able to break the windows. Given no other options, the employees entered the smoke-filled area and attempted to get to a stairway. Their bodies were found in the path of travel between the control center and the exit stairs.
When evacuation of personnel is likely to be delayed due to the need to initiate shutdown procedures, the most common life safety strategy is to provide one or more means of egress paths in which the available safe egress time (ASET) is greater than or equal to the required safe egress time (RSET). In this case, the required safe egress time includes the time necessary for the control personnel to initiate or, if necessary, complete plant shutdown procedures.
NFPA 101: Life Safety Code contains a requirement that ancillary facilities within a new industrial occupancy are to be provided with at least two means of egress arranged such that both egress paths are not likely to be compromised by a single fire event (NFPA 101-2015, Chapter 18.104.22.168.1). This general requirement applies to any ancillary facility, which could include administrative spaces, laboratories, and employee service areas.
The code also contains a requirement that if evacuation will be delayed, the ancillary spaces are to be separated from the industrial occupancy and at least one egress path shall be separated from the industrial occupancy by construction having a 2-hour fire-resistance rating (NFPA 101-2015, Chapter 22.214.171.124). Process-control centers are a common type of ancillary facility in which evacuation may be delayed to facilitate an orderly shutdown of the process or facility.
While the approach with ancillary spaces in industrial occupancies is to ensure that a safe means of egress will be available to occupants who may need to delay evacuation, there are other facilities in which the strategy is to "defend in place" or minimize the need to evacuate. This generally occurs in facilities providing medical care where moving fragile patients or residents could result in harm or in facilities needing to maintain security, which results in a desire to keep the occupants within the facility. In addition to requiring adequate staff ratios and training, the code requirements for these types of facilities address the construction, compartmentation, fire protection systems, fire prevention procedures, and emergency planning.
Whereas the scope of NFPA 101 involves life safety, the building construction type is not regulated for many of the occupancies addressed by the code. In these instances, the code presumes that adequate life safety can be provided without mandating specific types of construction. However, it is also recognized that the code presumes the building construction for those other occupancies will be addressed by building and fire codes (NFPA 101-2015, Chapter 1.1.6).
When a defend-in-place strategy is to be implemented, structural integrity is more critical. In such instances, NFPA 101 includes minimum construction types based upon the need to address the combustibility of the structural system as well as the fire-resistance rating. For example, NFPA 101, Table 18.1.6 requires that new health care occupancies be of noncombustible construction and have at least a 1-hour fire-resistance rating if the building is more than 1 story in height. Because buildings containing new health care occupancies must be protected throughout with an automatic sprinkler system, the construction requirements are in addition to the requirement for sprinkler protection.
In addition to structural integrity, such occupancies typically require some degree of compartmentation. With respect to the overall occupancy, NFPA 101 typically requires a fire barrier having 2-hour fire resistance to separate the mission critical occupancy from other occupancies in the building, when the separated occupancy approach is used. Unlike most other occupancies, the fire-resistance rating of the occupancy separation is not permitted to be reduced based upon the presence of an automatic sprinkler system. When compared with other occupancies with a similar fuel package, the mandate for the 2-hour separation is clearly intended to decrease the likelihood that a fire in an adjacent occupancy will impact the mission critical occupancy.
Within the mission critical occupancy, the compartmentation starts between the room and the corridor or common space. For example, NFPA 101, Table 22.3.8 contains specific requirements for separating the sleeping room within a detention and correctional occupancy from corridors and common spaces. As with health care occupancies, new detention and correctional occupancies are required to be protected throughout with an automatic sprinkler system, so the separation required is in addition to the protection provided by the sprinkler system. This separation provides protection for the occupant within the room of origin after they are removed from the room. It also provides protection for occupants in other rooms, thereby, reducing the need to relocate those that are not in the room of origin.
Should the fire continue to develop such that occupants outside the room of origin need to be evacuated, the mission critical occupancies are required to have smoke barriers, as indicated by NFPA 101. A smoke barrier typically has a fire-resistance rating of at least 1 hour and is designed to resist the passage of smoke. As such, moving patients, residents, or inmates across a smoke barrier should provide at least a temporary area of refuge, reducing the likelihood that those relocated will need to be evacuated to the outside or relocated vertically within the building. This horizontal relocation should require less staff than when evacuating the occupants or relocating them to other floors. However, it should also enhance the ability to continue to provide medical care in a health care occupancy or security in a detention and correctional occupancy.
In addition to regulating the travel distance to an exit, in the case of a health care occupancy, the code limits the travel distance to a smoke barrier. The travel-distance limit is based upon what the committee has determined to be an acceptable distance for staff to relocate patients across a smoke barrier. As such, considerations-such as the estimated time it will take to reach a smoke barrier as well as the physical challenges associated with relocating patients-have been considered. It should be noted that the travel distance to an exit door to the outside or an exit stairway is not to be considered when evaluating compliance with this requirement. The intent is that the occupants can be easily relocated to another part of the building on the same story.